textproduct: Rapid City

KEY MESSAGES

- Isolated severe thunderstorms possible Monday afternoon and evening, especially from the Black Hills east and south.

- Upper ridge will dominate the weather with subtle disturbances through next week with temperatures about 4-8F above normal

UPDATE

Issued at 1050 PM MDT Sun Aug 23 2026

Another round of at least isolated thunderstorms is expected to initiate off the Black Hills as early as 19z/1pm Monday afternoon, with the I-90 corridor and points south to the Nebraska border then favored through the evening as cells push southeast. Forecast soundings suggest this corridor will support the best combination of moisture, instability, and shear, with SBCAPE values potentially over 2,500 J/kg and convective temperatures likely to be reached to break the cap. Overall shear will be lower than today, and a mix of pulse, multicell, and transient supercell structures are possible. PoPs have been massaged to support this scenario based on the latest round of CAMs. Across most of the rest of the CWA, mostly dry and breezy conditions are expected. Fire weather conditions will be a touch elevated due to the heat and dry/breezy conditions, but wind gust and RH values are not expected to reach Red Flag criteria.

DISCUSSION

(This Afternoon Through Saturday) Issued at 1043 AM MDT Sun Aug 23 2026

16z surface analysis had frontal boundary from eastern MT to eastern CO and a trough from eastern CO into northeastern NE. Water vapour loop had upper ridge over the southwest CONUS with a shortwave over north-central WY. Overnight MCS about to fully exit the CWA with leftover low stratus burning off per 0.5km 1-minute visible imagery. Main concern this forecast will be effect of north-central WY shortwave.

This afternoon/tonight, shortwave moves into the Dakotas with slight increase in mid-level flow resulting in 30-45kt effective bulk shear later this afternoon. Td/s in the 60s will be sustained per breezy southeast winds ahead of frontal boundary. Band of 2-3KJ/kg MLCAPE forecast this afternoon over far western SD with minimum cap. Convective initiation (CI) will likely be over the Black Hills around 20z with supercells moving south/southeast at 10-20kts per Bunkers storm motion prognostications. Another area of strong/severe CI may develop over northwestern SD along the frontal boundary/trough interaction late this afternoon. CAMs handle this scenario in different fashion with some emphasizing MLCIN east of the Black Hills with some members not so much. Other strong/isolated severe convection may occur over the rest of the CWA. Sufficient buoyancy/bulk shear for marginal/slight (1-2 out of 5) of severe thunderstorms. Large hail/damaging wind the main threats with CIG1 magnitudes over western SD. Temperatures will be a bit below guidance this afternoon per low level moisture/dissipating stratus.

Monday, diurnally driven convection possible in the afternoon/evening, but the upper support less certain compared to today. Temperatures will be near guidance.

The rest of the forecast will be dominated by the upper ridge over the southwest CONUS. Ridge amplification into the northern Plains for the second half of the week suggests somewhat quieter and drier weather.

AVIATION

(For the 12Z TAFS Through 12Z Tuesday Morning) Issued At 518 AM MDT Mon Aug 24 2026

Thunderstorms are expected to form after 20z across the Black Hills, with additional development across NE WY and the west- southwestern SD plains through the afternoon and evening. Transient MVFR/IFR conditions are expected in any storms, although coverage of storms will be spotty. Outside of storms, VFR conditions will persist through the forecast period.

UNR WATCHES/WARNINGS/ADVISORIES

SD...None. WY...None.